RC filter cutoff calculator
A 1 kΩ resistor with a 1 µF capacitor gives a cutoff near 159 Hz, the −3 dB point where a low-pass RC filter starts cutting higher frequencies and a high-pass one starts letting them through. Free in your browser.
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Worked example
- Resistance R in ohms
- 1,000
- Capacitance C in farads
- 0.000001
- Filter type
- Low pass
- Cutoff hz
- 159.1549430919
- Angular cutoff rad s
- 1,000
- Time constant
- 0.001
The cutoff frequency of a first-order RC filter is the −3 dB corner where a low-pass stage begins to attenuate higher frequencies and a high-pass stage begins to pass them. For both topologies the analytic formula is one over two pi times resistance times capacitance, with R in ohms and C in farads, so a one kilohm resistor and a one microfarad capacitor land near one hundred fifty-nine hertz—the number every lab bench and audio crossover sketch needs before layout.
What the cutoff frequency of an RC filter tells you
An RC filter uses just two parts, a resistor and a capacitor, to let some frequencies through and weaken others. Put the resistor in series and the capacitor to ground and you have a low-pass filter; swap their places and it becomes a high-pass. In both cases the cutoff frequency is the −3 dB corner, the point where the low-pass begins to attenuate higher frequencies and the high-pass begins to pass them. It equals one over two pi times the resistance times the capacitance, so a 1 kΩ resistor with a 1 µF capacitor lands near 159 Hz. That is the number you need when sizing an audio tone control, an anti-aliasing stage or a filter that quiets a noisy sensor.
Units, zero values and checking it on the bench
Start with the units. Enter the resistance in ohms, so 1000 for a 1 kΩ part, and the capacitance in farads, using scientific notation when it helps: 1e-6 for 1 µF and 1e-9 for 1 nF. Typing 1 for a one-microfarad capacitor treats it as a whole farad, and the cutoff comes out far too low. A resistance or capacitance of zero is rejected, because the division would have no finite result. Choosing low-pass or high-pass only labels the result; the number stays the same. In lab work, students usually compute the cutoff first and then confirm the −3 dB point on the breadboard with a function generator and an oscilloscope.
What RC cutoff frequency means and how to choose R and C
A series resistor and a capacitor to ground form a first-order low-pass filter; swap the order and you get a first-order high-pass. In both cases the magnitude response crosses minus three decibels at f sub c equals one over two pi R C. Enter resistance as a finite number of ohms—for example one thousand for a one kilohm part—and capacitance in farads, using scientific notation when convenient such as one times ten to the minus six for one microfarad or one times ten to the minus nine for one nanofarad.
Zero resistance or zero capacitance is invalid input because the reciprocal diverges; negative magnitudes are allowed mathematically for signed symbolic workflows but practical design almost always uses positive values.
What you can do with it
Lab and homework verification
Students pick R and C for a low-pass or high-pass RC stage, compute fc, and confirm the −3 dB point before measuring the breadboard with a scope or function generator.
FAQ
What happens if resistance or capacitance is zero?
Zero R or C would make the cutoff formula diverge (division by zero), so those values are rejected and not charged.
Is the RC filter cutoff calculator free?
The browser tool runs free on your device.
What units should I use for R and C?
Resistance must be in ohms and capacitance in farads. Use scientific notation for small capacitors (for example 1e-6 for 1 µF, 100e-12 for 100 pF).
Is the cutoff formula the same for low-pass and high-pass RC filters?
Yes. A first-order RC stage uses one over two pi times R times C in both cases. A series resistor with a capacitor to ground is a low-pass; swapping the two parts gives a high-pass with the same corner.
What are the angular cutoff and the time constant in the result?
The angular cutoff is one over R times C, in radians per second, handy for transfer functions and Bode plots. The time constant is R times C, in seconds. With 1 kΩ and 1 µF they come to 1000 rad/s and 1 ms.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/elec/rc-filter-cutoff \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"resistance":1000,"capacitance":0.000001}'const res = await fetch("https://api.kit.forhosting.com/elec/rc-filter-cutoff", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"resistance": 1000,
"capacitance": 0.000001
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/rc-filter-cutoff",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"resistance": 1000,
"capacitance": 0.000001
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/rc-filter-cutoff", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"resistance":1000,"capacitance":0.000001}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"resistance":1000,"capacitance":0.000001}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/rc-filter-cutoff", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"resistance": 1000,
"capacitance": 0.000001
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.rc_filter_cutoff",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What RC cutoff frequency means and how to choose R and C
Optional filter_type accepts low_pass or high_pass so the JSON labels the topology you intend; the numeric formula is identical for first-order RC stages, so the field is documentation for your pipeline rather than a different solver path. Aliases R, ohms, C, and farads are accepted when your client already speaks those names.
Reading cutoff_hz, angular frequency, time constant, and units
Units are explicit: ohm, F, Hz, rad/s, and s. On success the response echoes resistance and capacitance with stable rounding, the filter_type label, and cutoff_hz as the primary result in hertz. Angular cutoff in radians per second equals one over the product R C and is exposed as angular_cutoff_rad_s for control-theory or Bode work that prefers omega over f. The time_constant field is simply R times C in seconds, the same tau that appears in step-response settling estimates and in related RC product calculators. The formula string is always fc equals one over two pi R C and method is always rc_filter_cutoff so downstream code can branch without sniffing shapes. Use cutoff_hz alone when sizing audio tone controls, anti-alias stages, or sensor anti-noise filters; use angular_cutoff_rad_s when matching continuous-time transfer functions H of s; use time_constant when you already think in tau and want consistency with step-response tables. Missing fields, non-finite numbers, out-of-bound magnitudes, and zero R or C all fail with invalid_input and are not charged after the engine rejects the request.
API usage, pricing, lab workflows, and practical limits
Call the free in-browser RC filter cutoff calculator for lab prep, lecture demos, and quick hand-check of pocket calculator results without installing SPICE or a spreadsheet. Send a JSON body with required resistance and capacitance, and optionally filter_type set to low_pass or high_pass. Use the prepaid API when a design automation script, curriculum generator, IoT firmware configurator, or educational platform needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance. Pair this endpoint with parallel capacitance or RL time-constant tools when the next step is combining passives or comparing RC versus RL dynamics, but keep the SEO intent separate: this page answers cutoff frequency of an RC filter, not total bank capacitance or inductive tau. Component values should stay within published magnitude bounds so Worker isolates remain safe; extreme products that overflow floating-point still fail closed rather than inventing a frequency. Authors shipping SDKs can map cutoff_hz and time_constant directly into UI fields students expect when they solve f sub c equals one over two pi R C by hand.
Integration use cases
Audio and sensor filter sizing
Designers size tone controls, anti-alias RC pads, and sensor noise filters by targeting a cutoff in hertz and iterating resistance and capacitance without opening SPICE for a first-order stage.
Firmware and curriculum automation
Config tools and lesson builders call a stable prepaid endpoint to return fc, omega_c, and tau as structured JSON for quizzes, calculators, and device configuration UIs.
Technical questions
How do I calculate RC filter cutoff frequency?
Send JSON with resistance in ohms and capacitance in farads (for example R: 1000, C: 1e-6). The response includes cutoff_hz from fc = 1/(2πRC), plus angular cutoff and the RC time constant.
Is the formula the same for low-pass and high-pass?
Yes for first-order RC stages. Both topologies use fc = 1/(2πRC). Optional filter_type only labels low_pass or high_pass in the response; it does not change the numeric formula.